On such extrusion apparatus there is produced cable of which the core is led through the bore of the central mandrel and surrounded successively first with a thin semi-conducting layer and later with a strongly insulating layer of non-conducting plastic material. With this extrusion apparatus, the individual mandrels or conducting pieces forming the forming nozzles are arranged stationary exactly centered. By reason of tolerances of the sheathing, cable produced on such extrusion apparatus does not meet the requirements of modern high voltage cable. The thickness of the sheathing around the core is not sufficiently uniform.
There is also known extrusion apparatus in which the mandrels formed as nozzles are divided and the end piece is arranged displaceably transversely of the axis of the mandrel. Through a displacement of the mandrel end piece during operation, the tolerance of the surrounding layers of the cable can be better adjusted, but the cable produced still has flaws, especially in the semi-conductive layer. This is attributable to the fact that the material to be extruded lodges in the adjustment places and here begins to polymerize or vulcanize and then during the operation, from time to time, is carried off by the flow of material and because of being already prematurely polymerized or vulcanized forms flaws in the semi-conductive layer. Such flaws must be cut out of the cable. This leads to considerable loss because not only the location of the flaws but also adjacent regions must be cut out. The resulting short lengths of the cable are frequently of limited value when they must be lengthened by being coupled bo other pieces of cable.